REAL-WORLD CONSIDERATIONS:Now, here's the "bad news:" With ANY SERIES (or out of phase) connection mod,IF the pickup's NEGATIVE LEAD has continuity with any metal parts - the cover or theframe - you MUST break this contact to isolate the metal cover or frame from the coil'snegative lead.WHY?Because, when you put the pup in SERIES - or out of phase - everything on the negativeside of the pickup's coil is now on the positive side of the guitar's circuit. That means thata metal cover or frame - and so, its mounting screws and springs, pole pieces etc - willhave continuity with the "hot" side of things. No arcs and sparks, smoke and flame, BUT,this may make the pickup more susceptible to hum, and it will certainly have the potentialfor unexpected and unwanted noises if you touch the pup or its screws while playing theguitar. If you accidentally touch it with a STRING, which is grounded, it will actuallySHUNT that pickup out of the tone (series) or mute the guitar (parallel).Because of this, I am in the habit of always recommending that if you just do the out ofphase mod, it is the NECK pup whose phase is reversed - most players are less likely totouch the neck pup than the bridge pup while playing. Also, if you are doing a series orout of phase mod to a Tele, it is FAR easier to mod the neck pup. Many Tele bridge pupshave an integral baseplate with continuity to their negative lead, and also require you toremove the bridge to get at them - too much work involved!SPECIFICALLY TELE:On your typical Tele neck pup, there is a metal cover. One of its mounting tabs, foldedunder the coil, usually has a small, short jumper to the eyelet on the pup's chassis thatconnects the coil to the negative lead. To "unground/reground" the cover, all you need todo is carefully clip that little jumper, breaking the connection, and then add a newINSULATED wire from the cover's tab back into the control cavity, where you willattach it to any ground. Please see the document “Unground/Regrounding a Tele.”SPECIFICALLY STRAT:On a stock-style Strat pup, luckily, this is seldom an issue. They are usually build on nonferrousframes, and have plastic covers - no worries about continuity with coil's the hot ornegative there. That's why I was able to so readily exploit all the series and series/out ofphase combos with my rotary switches.There IS a problem with some aftermarket custom Strat pups that are built with metalframes/chassis, and are wired up with only two conductors (leads). I simply do NOTrecommend the use of my switches - or that any series or phase mods - be attempted withpups built like that, as it has become popular to shield the underside of the pickguardaround the pickups. If the pup's metal frame has continuity to the negative lead, it willalso have continuity to ground through the mounting springs and screws touching theshielding foil on the back of the pickguard. Reverse the phase, or go series, on one ofthese pups, and you have an instant short circuit.MOST pup manufactures have wised-up to the mod market, and now, if using a metalframe, wire their pups THREE-CONDUCTOR - the hot lead, the negative lead, AND aseparate ground/shield wire (to ground the frame and/or cover) which has NO continuitywith the coil's negative lead.
SPECFICALLY HUMBUCKERS (and P90s):As for HUMBUCKERS, most modern/aftermarket FOUR-CONDUCTOR wired pickupsalso have a FIFTH conductor, which is typically a bare wire included inside the jacketedlead that will be the ground/shield connection to the chassis of the pup, and is completelyindependent and insulated from any leads from the coils. No worries, if this is the case.Where you run into the problem on humbuckers (and P90s) is if they are constructedusing a coaxial two-conductor lead with a braided/external shield, like the traditionalGibsons are made. The external braided conductor is typically soldered to the pup's metalchassis, and used as the ground/shield AND the coil's negative lead - yikes! Now youhave a pickup where not only the metal chassis, cover, and screws, but also the entirelength of the exposed braid on the lead has continuity with the ground of the guitar!These require some special treatment to do series and phase mods with.If you have an open-coil humbucker (no cover) or a P90, a simple trick is running alength of heat-shrink tubing over the entire length of the lead. This quick and dirtycompromise works, but the mounting screws and pole pieces are still "hot." It's less thanideal, but you CAN learn to live with it.However, this band-aid is even farther from ideal if your pickup has a metal cover. Now,you COULD, if you were an adventurous sort, remove the metal cover, go in andunsolder the coil's negative lead at the point where it is soldered to the pup's chassis andthe braided shield, and solder up a new negative lead for the coil (leaving the braidedshield soldered to the chassis as the ground conductor). That would certainly address allthe issues, but it's a long way to go and can be hazardous to the pickup and your wallet. Itis my strong recommendation that you should instead just shelf that old pickup intact,spend the bucks and get yerself a four-conductor pup to replace it.